Focus: Toward the Next Generation CD

By ‘writing’ and ‘erasing’ molecule-sized bits of a thin organic film, physicists have demonstrated its potential to become the next high-density storage medium.

From punch cards to floppy disks to CD-ROM’s, data storage devices continue to evolve. Researchers at the Oak Ridge National Laboratory (ORNL) in Tennessee don’t know what the next device will look like, but they believe they know what it will be made of: thin films of complex organic compounds. They report in the 21 February PRL that they have produced reversible changes in electrical resistance in molecule-sized regions of organic thin films. The results help pave the way for making thin-film storage devices because they mark the first time anyone has demonstrated reversibility–needed for “writing” and then “erasing” data–at a molecular level.

Other researchers have shown they could induce changes in conductance–the equivalent of “writing”–to a thin film, says Karl Sohlberg, a theoretician with the Oak Ridge group. But using only heat or laser pulses, they haven’t been able to “erase,” or reverse, the transition without clearing entire regions of the film, as if shaking clear a whole Etch-a-Sketch.

Sohlberg says organic compounds have piqued the interest of data-storage makers because of their incredible storage capacity. A typical CD-ROM, for example, has a storage density of perhaps 108 bits per cm2. The thin films used by the ORNL group and their colleagues at the Chinese Academy of Sciences in Beijing and the University of Chicago can store 1014 bits per cm2–a million-fold increase. Sohlberg says that organic-based data storage will ultimately create headaches for the engineers who have to design the machines fast enough to read from and write to such materials, but “that’s the engineering hurdle.”

The team made films of a complex of two organic molecules on a graphite surface. By applying a range of voltages with a scanning tunneling microscope, they found that the film underwent a conductance transition at 3.2 volts , where the resistance changed by a factor of 104. The transition took about 80 nanoseconds to occur.

The team reversed the transition by applying a reverse-polarity voltage pulse of -4.5 volts for 50 microseconds. They hypothesize that since the compound has a permanent electric dipole, the initial electric field (“write”) pulse causes a reorientation of the dipole and local disorder in the film, inducing a conductive state. “The molecules get all torqued, disoriented and twisted,” Sohlberg says. According to this theory, the reverse electric pulse re-creates the order and shocks the film back into a high-resistance state. The researchers tested this theory with tunneling electron microscopy and electron-diffraction studies and found, as they expected, that the nonconductive film was crystalline and that the conductive film was amorphous. The next step, says Sohlberg, is to seek out those materials which would maximize the change in conductance.

Tobin Marks of Northwestern University in Evanston, IL, says an important question that remains to be answered is whether the regions “written to” stay put. If over time diffusion smears out the localized regions of induced conductance, such thin films might lose their ability to store data. But overall Marks says the work is “promising.”